.div1 {
  width: 250px;
  height: 250px;
  background-color: red;
  margin: 40px auto;
}

.div2 {
  width: 0;
  height: 0;
  border-left: 125px solid transparent;
  border-right: 125px solid transparent;
  border-bottom: 125px solid red;
  margin: 0 auto;
}
